1. Field of the Invention
The invention is directed general to a method and apparatus for controlling diesel emissions for small to medium-sized mechanical handling equipment and particularly to a diesel particulate filter system for use with diesel powered forklift trucks.
2. Prior Art
Diesel engines are used in a variety of applications including forklift trucks for versatility, economy, safety and their characteristic low levels of gaseous emissions such as CO, CO.sub.2, NO.sub.x, SO.sub.x and hydrocarbons. The release of such pollutants into a working environment, even at relatively low levels is nevertheless a health concern, as is the emission of particulate pollutants (soot), which typically are present at a level of 1 to 2 g/m.sup.3 in diesel exhaust gas. Negative health effects of particulate emissions stem in part from the presence of potentially carcinogenic polyaromatic hydrocarbons.
Existing control technologies employed where diesel engines are operated in enclosed environments to reduce the emissions associated with diesel fuel combustion include ventilation, fume diluters, water scrubbers, catalytic purifiers and diesel particulate filters.
Catalytic purifiers act to substantially reduce the level of gaseous emissions and the liquid fraction of particulate emissions. Such devices incorporate a precious metal catalytic coating on pellet, ceramic, or metal substrates to convert CO and low molecular weight hydrocarbons to CO.sub.2 and water.
Diesel particulate filters are designed to eliminate 90% or more of diesel particulate as measured by the U.S. Federal Test Procedure. A filter trap comprising cellular ceramic elements is installed downstream of the exhaust manifold. When the quantity of trapped particulates is such as to cause the engine exhaust pressure to rise above a certain level, the particulates are burned off to regenerate the filter.
U.S. Pat. No. 4,899,540 (Wagner et al.) discloses the use of one or more ceramic filters for particulates in the exhaust gases of a diesel engine. A heating element is mounted on the intake end of each ceramic filter and regeneration is effected by turning on the heating element to radiate heat towards that end of the filter, turning on an air source to blow a low flow of combustion air through the filter and detecting the condition of regeneration and readiness for use by means of an arrangement of sensors.
U.S. Pat. No. 4,923,484 (Saito) discloses the removal of fine exhaust particles by the use of dual ceramic filters, with a mechanical valve arrangement and heating elements for alternately burning the particles from each filter.
Known emission control systems, by reason of their use of mechanical switching arrangements of varying degrees of complexity to divert the exhaust flow between individual members of a bank of diesel particulate filters, or their use of an external source of combustion air, do not lend themselves to easy retrofitting onto forklift trucks or like diesel machinery.